1. Field of the Invention
The present invention generally relates to digital cellular multi-pulse speech coders and, more particularly, to a fading bit error protection scheme for such coders.
2. Description of the Prior Art
Code Excited Linear Prediction (CELP) and Multi-pulse Linear Predictive Coding (MPLPC) are two of the most promising techniques for low rate speech coding. While CELP holds the most promise for high quality, its computational requirements can be too great for some systems. MPLPC can be implemented with much less complexity, but it is generally considered to provide lower quality than CELP.
Multi-pulse coding is believed to have been first described by B.S. Atal and J.R. Remde in "A New Model of LPC Excitation for Producing Natural Sounding Speech at Low Bit Rates", Proc. of 1982 IEEE Int. Conf. on Acoustics, Speech. and Signal Processing, May 1982, pp. 614-617. It was described to improve on the rather synthetic quality of speech produced by the standard U.S. Department of Defense LPC-10 vocoder. The basic method is to employ the Linear Predict Coding (LPC) speech synthesis filter of the standard vocoder, but to use multiple pulses per pitch period for exciting the filter, instead of the single pulse used in the Department of Defense standard system. The basic multi-pulse technique is illustrated in FIG. 1.
The digital cellular mobile radio environment exhibits a severe bit error effect known as the fading error. Fading errors occur when the moving vehicle encounters an area where the direct and reflected signals combine destructively and produce little or no signal level at the receiver. Such fades occur in a quasi-periodic fashion, where the time between fades and duration of fades depend on the vehicle speed, transmission rate, and carrier frequency. During a fade, all information is lost, and a random stream of bits is sent to the speech decoder. Thus, the speech decoder receives occasional bursts of an effective 50% bit error rate (BER). These bursts produce severe short-term "whoop" and "splat" artifacts in the output speech. Conventional error protection schemes (such as convolutional coding) cannot protect against most fades.
If a means for detecting fade occurrence is provided, then some degree of fade protection can be provided by taking "evasive action" within the speech decoding algorithm. Systems to accomplish such result take advantage of the quasi-stationary and periodic nature of the speech signal by interpolating or holding over spectral and gain information from a previous usable or "good" frame. Such system is described by N. Dal Degan et al. in "Communications by Vocoder on a Mobile Satellite Fading Channel", Proceedings of the IEEE International Conference on Communications, June 1985, pp. 771-775, for a standard LPC-10 vocoder. The Dal Degan et al. method detects fades using what they characterize as a "Signal Quality Detector" and by estimating the LPC distance between contiguous frames. Presumably, if the quality detector indicates an unusable or "bad" frame and the LPC distance measure between the "bad" frame and the previous "good" frame is above a threshold, the algorithm will reuse the previous frame's LPC coefficients; or, if a faded frame occurs between two "good" frames, it will interpolate the LPC values from the surrounding frames for the "bad" frame. While the Dal Degan et al. algorithm also interpolates (or holds over) values for the pitch period and the gain, it utilizes a signal level measurement, instead of a more exact error detection scheme, to indicate presence of a deep fade and does not provide adequate protection during periods of random pattern errors.
Methods are available to actually correct, rather than just detect, fading or burst errors. One method widely known employs Reed-Solomon coding on compact disc players for this purpose. While the performance of this code is very good, it has two drawbacks: 1) a very high overhead rate (i.e., high system complexity) is required for good performance under specified conditions, and 2) the fade length that can be corrected is limited. Drawback 2 becomes more important when high transmission rates are used and/or VHF carrier frequencies are used. For these reasons, the Reed-Solomon coding scheme may not be appropriate for Cellular or Public Service Trunked mobile radio.